The present invention relates to a carbon fiber, an electrode material for a lithium secondary battery, and a lithium secondary battery.
Among various types of secondary batteries, a lithium secondary battery is used as a power supply indispensable for information communications equipment represented by portable telephones and notebook personal computers, and contributes to reduction of the size and weight of mobile equipment.
Graphite or carbon fibers are used as an electrode material (additive) for the lithium secondary battery in order to provide the electrode with strength, conductivity, and the like.
A cathode material and an anode material for the lithium secondary battery have a layered structure. Upon charging, lithium ions are extracted from the cathode and intercalated between hexagonal carbon layers of the anode, whereby a lithium intercalation compound is formed. Upon discharging, a reaction occurs in which lithium ions are moved from the carbon anode to the cathode.
As described above, the carbon electrode material has a function of storing and releasing lithium ions. The quality of these storing and releasing functions greatly affects characteristics of the battery such as charge and discharge characteristics.
Graphite, in particular, anisotropic graphite has a typical layered structure and forms graphite intercalation compounds (GICs) when various types of atoms and molecules are introduced. When lithium ions are intercalated between the graphite layers, the electrode material (anode material, in particular) expands due to an increase in the gap between layers. If the charge and discharge cycles are repeated in such a state, the electrode may be deformed or lithium metal may be deposited, thereby causing capacity deterioration or internal short circuits. Moreover, if the gap between layers is expanded and contracted repeatedly, the graphite crystal structure may be damaged, whereby the cycle characteristics (lifetime) may be adversely affected. In addition, graphite exhibits inferior conductivity as the electrode material.
As a carbon material, a tube-shaped carbon fiber manufactured using a vapor growth process is known. In this tube-shaped carbon fiber, a plurality of concentric hexagonal carbon layers is stacked. In the case of using this carbon fiber as an anode material, lithium ions are intercalated only from the edges of the fiber, whereby sufficient lithium intercalation compounds are not formed. Therefore, a sufficient capacity cannot be obtained due to low electric energy density. Moreover, since the hexagonal carbon layers are in the shape of concentric circles, the hexagonal carbon layers are expanded by force when lithium ions are intercalated. This causes stress to occur, whereby the crystal structure may be destroyed.